In gas chromatography, components being analyzed are separated by injecting a precise amount of the gaseous or vaporized sample onto an analytical column. A carrier gas transports the sample through the column which is provided with a suitable packing material to provide selective separation of the sample components within the column. A suitable detector is provided to sense the tested for component or components as they elute from the column and to produce an appropriate output signal proportional to the component concentration in the sample. Various types of detectors are available for this purpose such as for example, flame ionization detectors, flame photometric detectors and thermal conductivity detectors. While each type of detector has certain advantages and disadvantages, thermal conductivity detectors have been found highly useful for industrial chromatography because they are easily maintained, safe to use and are highly sensitive to trace amounts of components.
Thermal conductivity detectors employ a heated metal filament or thermistor as a resistance element for sensing changes in the thermal conductivity of the test gas contacting the sensor. The sensor, which is heated to a predetermined temperature, is first contacted by the carrier gas which cools the resistance element at a given rate dependent upon the thermal conductivity of the carrier gas. Changes in temperature of the sensor are reflected both in its resistance and in the voltage and current required to maintain its temperature. As the test component elutes from the column and flows past the resistance element, the thermal conductivity of the test fluid which is different from the carrier fluid, will change the temperature of the resistance element which will again be reflected by changes in the voltage and current required to maintain its temperature. Such changes in current and/or voltage are recorded as a measure of the concentration of the test gas in the sample. It will be apparent, however, that thermal conductivity detectors are subject to several problems which can affect the results obtained using them. For example, thermal conductivity detectors are subject to a thermal lag as a result of heating and cooling the resistance element, and such thermal lag can result in loss of sensitivity and reduced response time for the detector element. Efforts to compensate for thermal lag have included reducing the dimensions of the resistance element and increasing the temperature at which the resistance element is operated. However, overheating of the resistance element can result in physical damage and ultimate destruction of the element. Replacement of the thermal conductivity element requires considerable maintenance and calibration of the equipment which may result in loss of production time if the apparatus is employed in monitoring an industrial process or the like. In addition to the foregoing, the design of the detector cell in which the resistance element is contained is also highly important since poor cell design can result in excessive heat loss from the resistance element and attendant erroneous results. Control of the temperature of the carrier and sample within the separation column is also highly important to prevent inadvertent condensation of the sample and a resulting change in its concentration as well as unwanted changes in the temperature of the atmosphere immediately adjacent the sensor due to the temperature of the carrier and sample. In this regard in certain prior art chromatographs, temperature control of the column and the cell has been accomplished by separately controlling the temperature of the detector block, the sensor and the separation column. With this approach, unless complicated circuitry and expensive control instrumentation are used, it is difficult to achieve close control of the respective temperatures of the detector block, the sensor and the separation columns. Fluctuation of these temperatures can result in instrument drift and loss of sensitivity. Efforts to control the column temperature and the temperature of the detector block using a single heater and controller have produced units of somewhat lower cost. However, such units generally fail to achieve the precise temperature control desired for best results and such units have been found unsafe to use with hydrogen carrier gas because of designed cost economics. This is highly undesirable as hydrogen gas is widely used as a carrier gas throughout the world because of its availability and desirable thermal conductivity characteristics.